Desmodromic valve systems for engine inlet and exhaust valves are well known and a sub-set of these mechanisms using a combined pull-push rod to actuate a valve is also long-established. Traditionally, these mechanisms have left a certain amount of lash between the opening and closing parts of the mechanism in order to avoid potential “fight” between the two actions arising either from tolerance errors or by changes in component dimensions with temperature. Such an eventuality could lead to rapid wear of the mechanism or a catastrophic failure due to the mechanism locking up. An exhaust valve can “grow” by 0.15 mm quite easily at full load.
Common past practice was to positively close the valve to within a few thousandths of an inch of the seat and then allow cylinder pressure to do the rest. FIG. 1 shows an example of a known desmodromic system. The figure shows two valves each of which is opened and closed by a respective pair of rocker arms which are in turn driven by opening and closing cams on a common cam shaft. As can be seen from the figure, the closing rocker arms are fitted with torsion springs acting around the axis of the closing rocker arms. However, these are helper springs which are used to suppress rattle noise and do not provide any significant closing biasing force on the valves.
More recently, emissions regulations have made this approach impractical and the valve now has to be closed using spring force. Ducati engine designs (using desmodromic valve systems) use a spring not dissimilar in terms of spring force from a conventional spring for this purpose. The issue here is that these springs act in parallel with the cam forces so the opening cam has to provide enough force to compress the spring as well as to accelerate the valve mass (see FIG. 2). This is a disadvantage because it increases the loads in the system and therefore the stresses, the system mass and the parasitic losses.
This approach is also unsuitable for independent valve actuation mechanisms in which, instead of a mechanical linkage between the engine output, an electromagnetic actuator is used.
For example in the case of the present applicant's electromagnetic valve actuation systems (as described in WO 2004/097184 and WO2011/061528), the additional torque required of the actuator in order to compress a spring in parallel with the valve mass is doubly undesirable. Not only would it require a significantly larger electrical actuator but the electrical energy demand would also be significantly increased, at the expense of the overall efficiency of an engine fitted with such a mechanism.
EP 2198129 (Pattakos) shows a desmodromic valve actuation mechanism in which a valve actuator exerts a closing force on a valve stem through an elastic washer which helps to ensure that the valve, when closed, is sealed against its seat. The washer is carried on the actuator so that the latter does work on the washer only when the valve is closed. However the mechanism uses a complex linkage for connecting the actuator to the engine output and various tolerances in that mechanism, and the fact that the valve, in effect, floats on the washer means that the actuator movement must be tightly constrained.
To that end the cylinder head of the engine is formed with an integral guide for the actuator, thus further increasing weight and complexity of the system.